The Valencia defender has stepped in for the former Liverpool man, who will miss the tournament through illness, though he is not guaranteed a spot on the final list of players
The former Liverpool stopper had been selected by coach Luis Milla as part of his provisional 22-man squad for the Games but has been forced to withdraw through illness.
The Spanish Football Federation have announced that Ruiz will take the Athletic man's place in the team, but he will continue to train with Los Che and will only be called to join the national squad if necessary.
Both Ruiz and San Jose were part of the Roja side that tasted glory in the European Under-21 Championship in 2011.
Milla will be disappointed to lose the former Liverpool defender, who had become somewhat of a cornerstone at the heart of the Spanish defence, but also because the news comes shortly after Barcelona midfielder Thiago pulled out of the squad through injury.
The Spain coach will have to cut his squad to 18 players before their Olympic opener against Japan at Hampden Park on July 26.